A testing tube of this type is known from German published patent application DE-OS 14 98 909. The known testing tube is used as a colorimetric gas dosimeter which contains a granular indicator layer within a glass tube. The glass tube, which can be opened at one end, is exposed to the gaseous toxic substance that is to be detected and a recognizable change in color is caused by diffusion of the gaseous toxic substance into the porous indicator layer when there is a corresponding impregnation of the carrier material. The advancing diffusion of the toxic substance into the indicator layer is indicated by the progression of the zone of altered coloration, which accordingly provides a measure of the quantity of toxic substance in the gas that is being tested. Gases that can be detected with gas dosimeters of this type are those which produce a color reaction directly with the indicator. Among these gases are hydrogen sulfide, sulfur dioxide, nitrogen dioxide and ammonia. Vapors of organic solvents, such as trichloroethene vapors, cannot be measured with this gas dosimeter tube.
However, U.S. Pat. No. 4,271,125 discloses that testing tubes intended for quantitative analysis, for instance of metal cyanides in aerosol form, are provided with a front layer of a carrier material impregnated with sulfuric acid or phosphoric acid, which with the metal cyanides forms a substance that is then detected by means of a color reaction with an indicator layer which follows the front layer. To detect the toxic substance that is to be investigated, a defined quantity of test air is passed through this known testing tube by means of a suction device, so that in the direction of the flow of testing air, the reagent layer must be arranged ahead of the indicator layer. If the air sample that is passed through the tube reaches the reagent layer first, then the toxic substance that is to be measured is converted into other reaction products, some of them gaseous. As the air sample continues to be aspirated through the tube, the gaseous reaction products leave the reagent layer and enter the following indicator layer, where they are converted into a colored reaction product and form a colored zone; at a given sample volume, the length of this colored zone is a function of the concentration of the toxic substance that is to be detected.
If the order in which the reagent layer and the indicator layer are arranged in the known through-flow testing tube were adopted for the known gas diffusion testing tube that can be opened at only one end, there would be a partial back diffusion of reaction products from the reagent layer to the opening of the gas diffusion testing tube, which would not produce a color change in the indicator layer that would be a function of the actual concentration. The back-diffused reaction products make no contribution at all to the color change in the indicator layer.